The subject of the present invention is a method of preparing phthalide, in which 3-chlorophthalide is subjected to a hydrogenolysis in the presence of hole metal catalysts.
The exchange of hydrogen for organically bound halogen, hereinafter to be referred to as hydrogenolysis, in the presence of palladium, platinum or nickel catalysts, is known. In this process, aromatically bound halogen is more easily substituted than aliphatically bound halogen. In all cases, the less acid the reaction solution is, the more easily the reaction takes place. However, since hydrogen chloride is liberated in the hydrogenolysis, it is common practice to add known hydrogen chloride acceptors to the reaction mixture to increase the speed of reaction and the volume-time yields.
The addition of acid-binding compounds, such as, for example, amines, sodium acetate, and alkali-containing methanol, has, however, the disadvantage that the working up of the reaction product and of the catalyst is thereby made considerably more difficult. In particular, the reprocessing of the catalyst important to the process is possible only by means of a plurality of steps, so that a procedure of this kind is not technically feasible.
Another possibility of intercepting the hydrochloric acid that is formed would consist in performing the hydrogenolysis in the presence of a solvent having a high ability to dissolve hydrogen chloride. A solvent of this kind for the hydrogenolysis of benzyl chloride to toluene is, for example, methanol.
If this procedure of hydrogenolysis in the presence of methanol is to be applied to the preparation of phthalide from 3-chlorophthalide, however, the following disadvantages are apparent: The primarily forming phthalide reacts immediately with the hydrochloric acid in the solution to cleave the lactone ring further to o-chloromethylbenzoic acid, which further reacts with excess methanol present to form toluylic acid methyl ester, so that this ester develops in this procedure as one of the main products, while phthalide is produced in a yield of only 15 to 18%.
Even if 3-chlorophthalide is subjected to the hydrogenolysis at, for example, 140.degree. C., under pressure, in the absence of methanol, the above-mentioned lactone cleavage occurs, the chloromethyl group being further degraded to the methyl group, so that the reaction mixture consists mainly of toluylic acid and not much phthalide.
The problem thus existed of performing the hydrogenolysis of 3-chlorophthalide such that the formation of undesired by-products would be suppressed and so that yields of phthalide greater than 90% would be obtained.